Mask smaller in Video Sequence Editor than in Movie Clip Editor
I found the reason for the mismatch of the mask: the input video in the movie clip editor is in 4K with 4096x2160 px and the output video in the video preview window is downscaled to fullHD with 1920x1080 px. Therefore the aspect ratio of the input (1.89 : 1) is different to the output video (1.77 : 1) hence a mismatch of the mask in the output video